feat: add method for blocking until shutdown is complete

[meowgorithm]

This feature introduces a Program.Done() method which will block until a Program has completely exited. This is useful when you want to wait until Bubble Tea has completely restored the terminal before printing output elsewhere in your Go application.

In particular, this feature can help prevent race conditions where Bubble Tea and other output are attempting to print to the terminal at the same time.

Consider the following:

// Draw a spinner
go func() {
	if _, err := p.Run(); err != nil {
		fmt.Println("Uh oh:", err)
		return
	}
}()

result := someLongRunningOperation()

p.Quit()   // stop the spinner
<-p.Done() // wait for cleanup

// Print safely.
fmt.Println("Okay, so the results are in:", result)

It may also make sense to include a convenience function which combines Program.Quit and Program.Done. Potentially named Program.Shutdown()?

[meowgorithm]

Closing in favor of #722, which has a much nicer API!